Cryogenic cooling of exothermic reactor

ABSTRACT

Method and apparatus for effectively and safely using cryogenic liquid to cool a reactor vessel housing an exothermic reaction wherein the cryogenic liquid is subcooled, valved into the reactor in a downward direction, and shielded by annular coaxial shielding gas upon injection into the reactor.

TECHNICAL FIELD

This invention relates generally to direct contact cooling and tocooling a reactor vessel housing an exothermic reaction.

BACKGROUND ART

It is important that a reactor containing an exothermic reaction becooled for safety reasons as well as to ensure that the reactionproceeds efficiently. Cooling is generally carried out by indirect heatexchange such as by maintaining the reactor in a water bath or in acooling jacket.

Direct heat exchange, wherein the cooling fluid directly contacts thecontents of the reactor, is more efficient than indirect heat exchangebut may be disadvantageous because the contact of the cooling fluid withthe reactants may adulterate or otherwise harm the product.

Cryogenic liquids contain a large amount of refrigeration and might beuseful for direct contact cooling of an exothermic reactor owing to thechemical inertness of many cryogenic liquids. However, because of theextremely cold temperatures characteristic of cryogenic liquids,freezing of the reactants, solvents and/or products of the reaction ontothe injection device which provides the cryogenic liquid into thereactor vessel creates a danger that the flow of cryogenic liquid intothe reactor vessel will slow or stop altogether. This will result in arunaway reaction with catastrophic consequences. Thus, the considerablepotential advantages of using a cryogenic liquid to cool an exothermicreaction by direct contact have not been realized.

Accordingly, it is an object of this invention to provide a system whichenables one to effectively employ a cryogenic liquid to cool a reactorcontaining an exothermic reaction by directly contacting the contents ofthe reactor with cryogenic liquid.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent to one skilledin the art upon a reading of this disclosure, are attained by thepresent invention, one aspect of which is:

A method for cooling a reactor containing an exothermic reactioncomprising:

(A) subcooling a cryogenic liquid;

(B) passing the subcooled cryogenic liquid through at least one valve;

(C) thereafter injecting the cryogenic liquid in a downwardly directedstream into a reactor vessel containing an exothermic reaction;

(D) providing shielding gas around the downwardly directed streaminjected into the reactor vessel; and

(E) cooling the contents of the reactor vessel by direct heat exchangewith the cryogenic liquid.

Another aspect of this invention is:

Apparatus for cooling a reactor containing an exothermic reactioncomprising:

(A) a subcooler and a reactor vessel;

(B) means for providing cryogenic liquid into the subcooler;

(C) means for passing cryogenic liquid from the subcooler to the reactorvessel, said passing means comprising at least one valve;

(D) an injector for receiving cryogenic liquid and injecting cryogenicliquid into the reactor vessel, said injector oriented so as to injectcryogenic liquid into the reactor vessel in a downwardly directedstream; and

(E) means for providing shielding gas around the downwardly directedstream of cryogenic liquid.

As used herein, the term "cryogenic liquid" means a liquid having atemperature at or below -80° C.

As used herein, the term "subcooling" means cooling a liquid to be at atemperature lower than that liquid's saturation temperature for theexisting pressure.

As used herein, the term "exothermic reaction" means a chemical reactionwhich is accompanied by evolution of heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of one embodiment of anarrangement for the practice of this invention.

FIG. 2 is a cross-sectional representation of one preferred embodimentof a nozzle which may be used to inject cryogenic liquid into thereactor vessel in the practice of this invention.

DETAILED DESCRIPTION

The invention is useful for controlling an exothermic reaction. Anexothermic reaction occurs when the heat content or enthalpy of theproduct or products is lower than that of the reactant or reactants, andheat is liberated.

Many oxidation reactions release heat during production of industrialchemicals such as adipic acid, cyclohexane and maleic anhydride.Free-radical or cationic polymerization reactions are typicallyexothermic. Heat has to be removed from polystyrene, polymethylmethacrylate and butyl rubbers during the bulk polymerization step.Alkenes, such as methyl butene, isobutylene, dimethyl butene and hepteneare made from catalytic hydrogenation under exothermic conditions.

The invention will be described in detail with reference to theDrawings. Referring now to FIG. 1, reactor vessel 1 preferably has anagitator 2 driven by motor 3 by means of connecting rod 4. Reactants arepassed through valve 5 in conduit 6 and into reactor vessel 1 whereinthey react in an exothermic reaction. The entire reaction mixture, e.g.reactants, solvents, catalysts, etc. could be provided into reactorvessel 1 as a mixture through a single conduit, as illustrated in FIG.1, or the components of the reaction mixture could be provided into thereactor vessel through a plurality of conduits, separately or in anycombination.

Cryogenic liquid is passed from a source (not shown), such as a storagetank, through conduit 7 and valve 8 into subcooler 9. Among thecryogenic liquids which may be used in the practice of this inventionone can name liquid nitrogen, liquid argon, liquid helium, liquid carbondioxide and liquid air. The preferred cryogenic liquid is liquidnitrogen. Generally the cryogenic liquid will be provided into subcooler9 at a pressure generally within the range of from 20 to 100 pounds persquare inch absolute (psia), although higher pressure may be used.

A subcooler is used to condition the liquid cryogen. The subcoolerillustrated in FIG. 1 is a heat exchanger fashioned as a tube within atube. In operation, a portion of the liquid cryogen, e.g. liquidnitrogen, under pressure from the central tube is metered into theannular space by a control valve. The annular space is maintained atessentially 0 pound per square inch gauge (psig) and therefore themetered fluid boils furiously until its saturated pressure reaches 0psig. This boiling cools the annular liquid to a temperature lower thanthe liquid in the inner tube. By heat exchange with the colder annularfluid, the liquid nitrogen under pressure in the inner tube is subcooledbelow its saturation temperature. Any vaporized nitrogen (due to heatleak or pressure drop) in the inner tube will be condensed. The vaporgenerated by boiling in the annular space is vented to the atmosphere.

Subcooled cryogenic liquid is withdrawn from subcooler 9 through conduit10 and is passed through at least one control valve wherein the liquidcryogen flow rate is regulated according to the refrigeration demands.The pressure of the liquid cryogen is reduced generally by at least 0.5psi. In the embodiment of the invention illustrated in FIG. 1, subcooledcryogenic liquid is passed through control valve 12. When there is norefrigeration demand, control valve 12 is capable of shutting off theliquid cryogen flow completely.

A second valve upstream of the control valve is a shut-off valve atwhich loss in cryogen gas pressure or electrical power will cause valve11 to shut off the liquid cryogen flow completely.

The resulting cryogenic liquid is then injected into reactor vessel 1 ina downward direction. By "downward" or "downwardly" it is meant within45 degrees of vertical. In the embodiment illustrated in FIG. 1, thecryogenic liquid is passed into reactor vessel 1 through verticallyoriented injector or provision means 13 which comprises a centralpassage and an outer passage coaxial with the central passage. Thecryogenic liquid is passed through the central passage of provisionmeans 13 while shielding gas, which has been provided into provisionmeans 13 by means of conduit 14, is passed through the outer passage ofprovision means 13. Generally and preferably the shielding gas will havethe same composition as the cryogenic liquid. The preferred shieldinggas is gaseous nitrogen.

A backup gas, preferably gaseous nitrogen at room temperature, issupplied through conduit 19. Conduit 19 is connected immediatelydownstream of the control valve 12. The backup gas is operating at apressure at least 5 psi lower than the liquid cryogen at the exit pointof the control valve 12. The pressure of the backup gas is controlled bya discharge pressure regulator 20. Under normal or maximum coolingconditions, the backup gas will not be flowing since its pressure islower than the liquid nitrogen in conduit 10. When the control valvereduces the liquid nitrogen flow to a point where the pressure at theexit of the control valve 12 is lower than the gaseous nitrogen inconduit 19, the backup nitrogen will start flow. This arrangement willkeep the reactant from entering the cryogenic nitrogen system during thecontrol actions of valve 11 or valve 12.

The cryogenic liquid and the shielding gas proceed through provisionmeans 13 through the central and outer passages respectively and areinjected into the interior volume of reactor vessel 1 preferably throughinjection nozzle 15 of injector 13. The cryogenic liquid is injectedinto the interior volume of reactor vessel 1 in a downwardly directedstream and the shielding gas is provided around this downwardly directedstream and for along at least a portion of its length.

The cryogenic liquid injected into the reactor vessel directly contactsthe contents of the reactor vessel, e.g. reactants, solvents, products,etc., as well as the interior surface of the reactor vessel, and coolsthe contents and the reactor vessel itself by direct heat exchange. Thedirect heat exchange causes the cryogenic liquid to vaporize and thisvaporized fluid as well as the shielding gas is passed out of reactorvessel 1 though conduit 16. The cooled reactor contents including theproduct are withdrawn from reactor vessel 1 through conduit 17 and valve18 and the product is recovered. Preferably the reactor vessel isagitated such as by agitator 2 illustrated in FIG. 1 as this serves tobetter disperse the cryogenic liquid within the reactor vessel enablingbetter heat exchange and the avoidance of cold spots.

The subcooling of the cryogenic liquid enables the cryogen to bedelivered to and injected into the reactor vessel essentially withoutany vaporization. This enables the control valve to dispense the correctproportion of coolant for the exothermic reaction. A large percentage ofvaporized cryogen can create vapor locking of the transfer line and theinjection nozzle, resulting in uncontrolled exothermic reactions orrunaway reactions.

The backup gas and the shielding gas enable the avoidance of reactorcontents freezing upon the injection device and clogging the devicewhich has heretofore hampered the effective use of cryogenic liquid forcooling exothermic reactions. The greater mass and consequent force ofthe liquid cryogen stream forces away any liquid droplets or solidparticles that might otherwise freeze onto the injector. The shieldinggas serves as a physical barrier to ensure against any buildup of frozenparticles from the periphery clogging the injector.

The downward injection direction is also important in achieving thebeneficial results of the invention. As the cryogenic liquid isvaporized, a portion of the resulting vapor rises back up and forms aprotective gas bubble or barrier around the tip of the injectorproximate where the cryogenic liquid is injected into the reactorvessel. Cryogenic liquid passes easily downward through this protectivebubble but the bubble forms a barrier serving to keep the reactor vesselcontents from contacting and freezing upon the injector.

Thus the subcooling, shielding gas and downward injection work inconcert to enable cryogenic liquid to be employed for cooling exothermicreactions without encountering clogging of the cryogen provision meansby frozen rector vessel contents which results in reduction or totalloss of coolant and a runaway reaction.

FIG. 2 illustrates a preferred embodiment of a nozzle which may be used,such as nozzle 15 of FIG. 1, to provide both cryogenic liquid andshielding gas into the interior of the reactor vessel. Referring now toFIG. 2, injection nozzle 50 can be attached to the cryogen provisionmeans by means of threads 51. Nozzle 50 comprises central section 52which communicates with and thus extends the central passage of thecryogen provision means and which receives cryogenic liquid forinjection into the interior of the reactor vessel at cryogenic liquidinjection point 53. Central section 52 has an upstream section 54, whichhas a diameter similar to that of the central passage, and a downstreamsection 55, which has a diameter smaller than that of upstream section54.

Nozzle 50 also comprises outer section 56 which forms a passage coaxialto central section 52, which communicates with the outer passage of andthus extends the cryogen provision means, and which receives shieldinggas for injection into the interior of the reactor vessel at shieldinggas injection point 57.

The injection end or face of nozzle 50 is in the shape of an invertedcone. In this way cryogenic liquid injection point 53 is lower thanshielding gas injection point 57 so that the shielding gas enters theinterior of the reactor vessel before, i.e. upstream of, the cryogenicliquid. That is, injection point 53 is closer to the apex of the cone ofthe face of nozzle 50 than is injection point 57; preferably, asillustrated in FIG. 2, injection point 53 is at this apex. This ensuresthat the cryogenic liquid injection point is fully shielded at alltimes.

Now by the use of this invention one can effectively and safely usecryogenic liquid to cool a reactor containing an exothermic reaction.Although the invention has been described in detail with reference tocertain preferred embodiments, those skilled in the art will recognizethat there are other embodiments of the invention within the spirit andthe scope of the claims.

We claim:
 1. A method for cooling a reactor containing an exothermicreaction comprising:(A) subcooling a cryogenic liquid; (B) passing thesubcooled cryogenic liquid through at least one valve; (C) thereafterinjecting the cryogenic liquid in a downwardly directed stream into areactor vessel containing an exothermic reaction; (D) providingshielding gas around the downwardly directed stream injected into thereactor vessel; and (E) cooling the contents of the reactor vessel bydirect heat exchange with the cryogenic liquid.
 2. The method of claim 1wherein the cryogenic liquid comprises nitrogen.
 3. The method of claim1 wherein the shielding gas comprises nitrogen.
 4. The method of claim 1wherein the direct heat exchange vaporizes the cryogenic liquid and atleast a portion of the resulting vaporized fluid rises and forms a vaporbarrier proximate where the cryogenic liquid is injected into thereactor vessel.
 5. The method of claim 1 wherein the cryogenic liquidstream is injected into the reactor vessel lower than where theshielding gas is provided into the reactor vessel.
 6. The method ofclaim 1 further comprising making back up gas available to the subcooledcryogenic liquid prior to where the cryogenic liquid is injected intothe reactor vessel, said back up gas being at a pressure lower than thepressure of the subcooled cryogenic liquid.